1. Field of the Invention
This invention relates in general to wellhead equipment for oil and gas wells, and in particular to an emergency casing hanger.
2. Description of the Prior Art
In a well of the type concerned herein, a wellhead housing will normally be located on land, or on a surface platform, if offshore. The wellhead housing has a landing profile or shoulder within its bore. When running casing, the casing is lowered into the well. A casing hanger is installed on the upper end of the string of casing. This casing hanger lands on the landing shoulder in the bore of the wellhead housing.
After cementing, a packoff is positioned between the casing and the wellhead housing. This packoff locates between machined surfaces on the wellhead housing and the casing hanger.
Occasionally, the casing will not smoothly proceed to the bottom of the well. When this occurs, the casing hanger will not be properly positioned to land in the wellhead housing. Generally, when this happens, the casing cannot be retrieved to the surface and becomes stuck. In that case, the casing must be cut above the landing shoulder after cementing. In the prior art technique, the assembly is supported by slips in the wellhead housing.
One problem with supporting a casing stub in an emergency procedure occurs because the casing will often not be centralized within the wellhead housing. The casing particularly will be off center in deviated wells. Conventional slips, which move down a bowl by gravity, will not center the casing. If the casing is not centered, it will be difficult to seal the space between the casing and the wellhead housing. This is particularly a problem with metal-to-metal seals.
Another problem with sealing against a casing stub occurs because the casing does not have a smooth machined surface for receiving a packoff. The casing outer diameter has a high dimensional variation. The outer diameter may be slightly oval shaped. The surface of the casing may have many defects, such as rust, pock marks, and tong marks.